1. Field of Invention
This invention relates to the analysis of liquid samples by transmission spectroscopy, specifically to a flow cell with an improved flow of sample.
2. Prior Art
Optical transmission liquid cells are well known and widely utilized in measuring the optical properties of liquid samples. The general configuration of these cells includes two substantially parallel optical windows spaced a certain distance apart thus forming a gap between them into which a liquid sample can be introduced. A cell body holds windows in place in a leak proof way and usually incorporates an inlet and an outlet for the sample.
The windows of the cell are made of a material that is transparent to light in the spectral region of interest. For visible light the most commonly used window material is glass. Quartz is the material of choice for the ultraviolet spectral region. A large selection of materials, such as NaCl, KBr, CaF2, etc., are used in the infrared region. The window material is selected for optical transparency, chemical compatibility with the sample of interest, mechanical strength, temperature range, refractive index, etc. The gap between the two windows establishes the pathlength of light through the sample. As light passes through the sample, different wavelengths of light are absorbed differently by the sample. This selective absorption of light by a sample is a unique characteristic of the sample and can be used to identify unknown samples, quantify the concentration of a sample in a mixture, elucidate molecular structure of unknown samples, etc. The cell can be used in a static configuration where the cell is filled for analysis, for instance with a syringe, and the sample is stationary during the analysis, or in a flow through configuration where the sample is flown through the cell for analysis.
The optical pathlength is the most important parameter of a transmission cell. The pathlength determines the strength of absorption of light by the sample in the cell. The longer the pathlength through the sample the stronger the absorption of light by the sample. It is generally known that the optimum cell pathlength for a particular sample is one that provides for an absorption of about 63% of the incident light. In the visible and near infrared spectral regions most liquids absorbs light weakly and transmission cells in that spectral region employ longer pathlengths through the sample, 10 mm being the most common pathlength used. Such pathlengths do not present a substantial resistance to the flow of liquid through the cell. Thus transmission flow cells having longer pathlengths (generally more than 1 mm) are very common.
However, the mid-infrared spectral region, also known as the fingerprint region, which is characterized with strong sharp spectral absorption peaks highly characteristic of the sample requires submillimeter pathlengths, thus high flow rates through such cells are not possible. For instance, a 100 μm pathlength is typical for transmission cells that operate in the infrared spectral region. Clearly, very short pathlengths impede the flow of liquid between the two windows and particularly so for viscous samples. However, even for less viscous samples the narrow space between the windows becomes a great impediment to the flow of sample through the cell. In addition to slowing the flow of liquid through the cell, a short pathlength also makes such a cell vulnerable to clogging by particulates in the liquid. This greatly reduces the usefulness of transmission cells with submillimeter pathlengths for routine fluid monitoring applications where a flow-through operation is essential.
To monitor liquids using a transmission cell, usually a slip stream is set up to flow the liquid through the cell for analysis. Thus the liquid must travel through the slip steam from the intake point to the cell. To perform a spectroscopic analysis of the liquid present in the cell, light is passed through the cell and the absorption of light by the sample in the cell is measured. The result of the analysis reflects the state of the liquid at the intake point as it was at the time it was extracted into the slip stream. If the flow of sample through the cell is slow, the information obtained by the analysis significantly lags behind the status of the liquid at the point of extraction. While preferable to grabbing a sample and analyzing it offline, in most situations this delay is still undesirable, and potentially very costly, thereby limiting the utility of the analysis. Thus, if possible, it would be extremely useful, for those cases where the required pathlength of the transmission cell has to be in the submillimeter range, to have the cell constructed in such a way to enable a high flow of sample through the cell. One way to improve the flow through the bypass loop, known in the art, is to add a secondary bypass into the main bypass line very close to the cell that shunts most of the liquid flow around the cell leaving only a small fraction of the flow to travel through the cell. This secondary bypass with slow flow, as dictated by the cell, can be made close to the cell thus shortening the path along which the flow of liquid is very slow. The overall liquid flow through the bypass loop consists of the fast flow through most of the bypass loop and a short section of slow flow through the secondary bypass. This helps reduce the delay caused by the transit time of liquid from the intake point to the cell.